1. Field of the Invention
This invention relates to a semiconductor photosensor, and more particularly to a semiconductor photosensor having a spectral sensitivity characteristic close to human eye spectral sensitivity.
2. Background Art
Semiconductor photosensors are increasingly installed in information equipments for input and display of image information such as portable information equipments, liquid crystal TV sets, and digital cameras. For example, portable information equipments such as mobile phones, notebook personal computers, and PDAs (Personal Digital Assistants) are equipped with backlight for liquid crystal or other display screens and/or keypad illumination. In this situation, the brightness of semiconductor light emitting devices such as LEDs (Light Emitting Diodes) can be controlled in response to the ambient darkness to improve visibility and reduce power consumption. Taking a mobile phone as an example, in the daytime or in a well-lighted room, keypad LEDs are turned off and the brightness of backlight of the transmissive liquid crystal display is increased. On the other hand, in the outdoors at night, the keypad LEDs are turned on and the backlight of the liquid crystal display is reduced.
In addition, when video is displayed on a liquid crystal TV set or a mobile phone, it is desirable to use different gamma values for correction depending on the usage environment in order to improve visibility of the screen. To achieve this, semiconductor photosensors are required to have a spectral sensitivity characteristic in close agreement with human eye spectral sensitivity. Note that the semiconductor photosensors used for these applications are sometimes referred to as “illuminance sensors”.
A technology intended to achieve a spectral characteristic in close agreement with human eye spectral sensitivity is disclosed in Japanese Laid-Open Patent Application H8-330621 (1996). According to this technology, two light receiving means are provided. One of them receives light via an optical filter, which has the function of transmitting infrared light (blocking visible light). Therefore, subtraction between the detection outputs of the two light receiving means can be used to obtain a spectral sensitivity characteristic in the visible light region, that is, close to human eye spectral sensitivity.
Recently, however, sophisticated brightness control such as in video display requires a spectral sensitivity characteristic in closer agreement with human eye spectral sensitivity.